Added as_network and preparation for CRAN submission

[jhollway]

Description

Package

• Closed Submit to CRAN #21 by elaborating DESCRIPTION file in preparation for CRAN submission
• Updated several old URLs in documentation

Classes

• Closed Add sna integration #85 by adding as_network() to coerce objects into network class
• Modified other coercion functions to also work with network class objects

Checklist:

• PR form
  • I have given this pull request an informative title
  • Description above itemizes changes under subtitles, e.g. "## Collection""
  • Any closed, fixed, or related issues are referenced and explained in the description above, e.g. "Fixed #0 by adding A"
  • Package builds on my OS without issues
• PR checks all pass for latest commit
  • Package builds on Mac
  • Package builds on Windows
  • Package builds on Linux
  • CodeCov check: Package improves or maintains good test coverage
  • CodeFactor check: Package improves or maintains good style
• Documentation
  • Any new or modified functions or data have roxygen style documentation in their .R scripts
  • Any longer functions are commented inline so that it is easier to debug in the future
  • PR description above and the NEWS.md file are aligned
  • DESCRIPTION file version is bumped by the appropriate increment (major, minor, patch)